Document issues with default docker userspace proxy and large numbers of ports

[kristiankielhofner]

For the WebRTC dynamic media port range the default docker daemon settings will launch $NUMPORTS of userspace docker UDP proxies - which takes a long time and is a waste of resources. We should dramatically warn when they don't have /etc/docker/daemon.json:

{
    "userland-proxy": false,
    "iptables": true,
    "runtimes": {
        "nvidia": {
            "args": [],
            "path": "nvidia-container-runtime"
        }
    }
}

We may be able to get this via the Docker API but needs more research.

We should also update the default port range to ~10 ports or something so even if they miss the warning, don't know how to do it, or don't need more concurrent clients even default userspace UDP proxies aren't a big deal.